So you think if I would put a thinset between it would work and even be stronger? Theoretical it makes sense.

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You need all four sides of cement board secured with studs or blocking. With this in place I would bet that if you nailed off the first layer with some smaller nails and then installed the second layer with a slurry coat of something like Laticrete 254 your net result would be stronger than a 1/2" layer of Hardie. Why not buy one sheet of 1/2" and do a small test.

Set up a deflection test. Might save you some money if you can find out it works.

The board should not flex inward more than 1/4" between 16" on center walls studs.

L/360 deflection for a 16" center wall stud is very low under 1/16". This is very hard to meet with any backer board.

I would mock up a test and then add 100 lbs of weight on your mock up. Measure the sag. Then check the sag on a 1/2" board. If you are within these ranges I would think your fine.

What type of waterproofing are you planning to use over this install. If your skipping any waterproofing then having fuller sheets and less seams would be the way to go...

So the project at this point is just the shower. What kind of shower drain are you using? Is the shower base ready made or are you making a custom one?

Adding waterproofing to the job you can easily expect to pay another $60-90+ to waterproof the walls from the pan to about 6'. Typically 3'x3' shower needs one gallon on liquid membrane or one unit of Ardex 8+9 to waterproof it.

I don't think you need a waterproof wall. There will be not water standing on the wall for a longer time (gravity).
I will add as a barrier 15 roof felt behind the backerboard. For the pan I got the membrane. Shower drain is a Luxe which I bought when I still had a job. lol.
I am making an own pan.

Physically a 15 roof felt between the studs and the cement board should be fine. If water runs behind grout and board it will be stopped by the felt and the liner of course.
A membrane over the cement board is of course the better way or top of the line, but a felt behind will work also. Adding a membrane to the felt and leaving the cement board between like sandwich style, creates moisture.
Correct me if I am wrong.

MAd King your approaches are within industry guidelines and allowed by building codes. It's just your approach is the bare minimum approach. I can not answer your question with a simple one liner since there are so many variables. If your going to piece meal a load of scrap board you are incorporating a lot more area to absorb water.

I would not go this route but rather the topical membrane approach.

I wish you well and hope your shower is a success. I will not personally say "Great" sounds like a winner since it is not how we build our showers. I would remind you that you have already saved the money a container of of Ardex 8+9 or a bucket of Hydro Ban costs. That money could make your bathroom's shower waterproof - not water resistant. If you want to follow the base grade standard this of course is your choice but if you go that route I would be using full sheets of backer board. So your back to the start.

I was able to return the 1/4 hardibacker and got the 1/2 now. I didn't know that HomeDepot accepts returns after 90 days, but they kept the taxes, which I don't mind.
For the waterproofing, can I also use Redguard from HomeDepot? I would have to order Ardex online, because I can't find it locally.

I was able to return the 1/4 hardibacker and got the 1/2 now. I didn't know that HomeDepot accepts returns after 90 days, but they kept the taxes, which I don't mind.
For the waterproofing, can I also use Redguard from HomeDepot? I would have to order Ardex online, because I can't find it locally.

Thank you

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This sounds like a better plan.

When you install the backer board tape the seams and skim coat (flat trowel) the entire board. My testing has shown to me that the liquid membranes hold better over the thin-set than they do the backer board. Give the thin-set a few days to set up or use a self curing rapid setting thin-set and wait a day.

That is a good idea with the thinset. I also read that the Redgard can be thined upt to 50% for the first coat and then the second coat regular.
They also say that with Redgard a non-modified thinset is recommended.

That is a good idea with the thinset. I also read that the Redgard can be thined upt to 50% for the first coat and then the second coat regular.
They also say that with Redgard a non-modified thinset is recommended.

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Not sure I would do this. I have read that the incorporation of water to liquid membranes adds tiny air pockets and weakens the product. I'm doing a series of shear tests with uncoupling membranes and sheet membranes this week and later will be putting the liquids head to head. I'm going to see what affect the addition of water has on these when I do. I will at the same time test the varies backer boards at the same time for shear strength. If you can hold on a few weeks I'll have an answer for you. To be safe clean the backer board to remove any bond breakers and then burn in a good scratch coat of thin set.